Materials generally exhibit a certain band gap related to the material's physical and electronic properties. A band gap is specific to each solid material, and may be defined as an energy range in which there exists no electron state for placement of electrons. The band gap is described herein in terms of the energy difference (in electron volts eV) between the valence band and the conduction band for a material. The lower the band gap, the easier it is to ionize a material, e.g., by removing an electron.
High band gap materials, particularly semiconductors, are useful in photoconductive semiconductor switch (PCSS) applications. How high the band gap must be before the material is considered a “high band gap” material depends on what application the material is being used for. Generally, a high band gap material can be considered any material having an electronic band gap larger than about 1.0 eV. However, in some application, materials with an electronic band gap of larger than about 2.0 eV or more may be considered high band gap materials.
In addition to a high band gap, materials that are useful in PCSS applications have high critical electrical strength, high electron saturation velocity, high thermal conductivity, and low resistance when excited by a laser, or other optical source, with the proper wavelength and power.
Obtaining suitable PCSS materials presents certain challenges. For example, the purchase of PCSS materials is difficult as these materials are very expensive. Moreover, conventional formation techniques of suitable PCSS materials typically involve growing single crystals from seed crystals to a desired size and purity prior to cutting the single crystals into the desired shapes and size for photoconductive switches. This single crystal growing process, however, is extremely slow, costly and requires high formation temperatures, which limits the quantity and type of doping agents that may be dispersed in the molten single crystal growing process. Additionally, the crystal growing process is difficult to control, and frequently leads to crystal boules with significant imperfections, such as “pipes,” inclusions, impurities, and/or other defects, which reduce the useful yield of the boule itself and yields a final crystal product with less than desirable performance characteristics, especially in optical applications, such as PCSS applications.
Conventional methods for forming PCSS materials, with laser optical transparency, from nanopowders also present several disadvantages. Such processes typically start with a very pure co-precipitated powder, which is then slip cast in the presence of a gelling agent to form the green structure prior to sintering. A uniform slurry of high purity powder is poured into a plaster mold, which sucks the water out of the slurry by capillary forces and produces the green structure after drying. Using fluid flow and surface tension to consolidate the ceramic powder allows parts to be made with a uniform powder packing. However, because the mold removes the water, slip casting can only be used for relatively thin parts. The need for a very porous surface on the mold also introduces another variable in the green structure fabrication. The porous mold, usually made of commercial gypsum, may also be a source of contamination. Moreover, the presence of the gelling agent, or its by products, in the final structure is an impurity that adversely affects the optical properties of the ceramic. Cold uniaxial pressing and cold isostatic pressing have also been used to make transparent parts. However, inter-particle friction during the pressing process tends to prevent densification in the center of the part so that size of the part must be kept small enough that this does not cause porosity.
Finer nano-sized powder than that produced by precipitation may be used. This can be especially important for achieving high transparency needed for lasers. Finer particles, because of their increased surface area, sinter more easily. However, smaller nano-sized particles behave differently than larger (such as micrometer) sized particles during green structure consolidation. For instance, smaller particles experience more friction as they move past one another in a die making it more difficult to produce a uniform structure through cold pressing, especially where larger parts are desired. The higher surface area of finer particles also requires more water for wetting making it difficult to get the solids loading high for slipcast slurries. As a result, after slip casting there is significant shrinkage on drying, often leading to cracking and other problems. Finer particles are more susceptible to surface-area-dependent chemical reactions, as may occur between a porous mold and certain ceramic powders.
Since it is difficult to find all of the desired properties in a single material which can be used in PCSS application in a cost efficient way, it would be desirable to have methods to make materials that can be used in PCSS applications and/or to have additional materials capable of being used in PCSS applications that can be manufactured and/or produced more inexpensively and precisely than conventionally used materials.